This invention relates to an improved vibrating wire strain meter and an installation/retrieval tool therefor.
Strain meters of this type are well known and generally
involve the use of a metal sleeve across the bore of which is strung a taut steel wire. The wire can be caused to vibrate by means of an electromagnetic transducer which is also housed within the sleeve. The frequency of vibration depends among other things upon the length and tightness of the wire and thus frequency can be measured by the same transducer which causes the wire to vibrate. In use, the meter is mounted securely within a hole bored within the rock formation the movement of which is to be monitored. Movement of the rock formation causes deformation of the sleeve which causes the tightness of the wire to increase or decrease and hence the vibration frequency to increase or decrease. The frequency which is displayed on a remote readout unit is, therefore, a measure of the strain.
U.S. Pat. No. 4,159,641 (Hawkes), incorporated herein by reference, is typical of the kind of device which has been used in the past. Although the device described in that patent is a stress meter it is in fact measuring movement of rock formations and is, therefore, primarily a strain meter. The manner in which the metal sleeve or proving ring of Hawkes is secured within a borehole is by jamming a wedge assembly between a flattened top surface portion of the proving ring and the wall of the hole. Another important aspect of the Hawkes device is that the one end of the vibrating wire is located adjacent the flattened surface and the other end is located diametrically opposite that. In other words the ends of the wire more or less coincide with the areas of contact between the proving ring assembly and the borehole wall.
In common with other devices commonly in use the Hawkes device exhibits serious defects in operation the most significant of which is lack of accuracy or repeatability. In studies carried out by Richard Lingle and Philip H. Nelson and published in chapter 84 of Issues in Rock Mechanics published by the Society of Mining Engineers of the American Institute of Mining, Metallurgical and Petroleum Engineers, Inc., New York 1982 and incorporated herein by reference, it was concluded that this type of device "did not function well enough to provide reliable data on stress changes." The main problem seemed to be that the device was unduly sensitive to the manner in which it was seated within the borehole. The problem is so great that the same device reinstalled several times in the same place in the same hole gave entirely different readings. More particularly, deviations as great as .+-.20% from the manufacturer's calibration factor were found.